Additives are often used to impart polymeric materials with certain desirable properties. For example, electrically conductive fillers can be used in applications where static dissipation or greater degrees of electrical conductivity are important. Other fillers can be used to increase the permeation resistance of a material to certain gases or liquids. However, in some cases, such as at loadings required to optimize the desired properties (for example, to go over the percolation threshold for electrical conductivity), the use of such additives can be detrimental to other desired characteristics (such as mechanical properties) of the materials and/or can be costly. However, it is often not necessary that the additives be dispersed uniformly throughout the material. For example, when electrical conductivity is needed, it is often only necessary that the portions of an articles near its surface be conductive. Furthermore, when additives that increase the permeation resistance of a polymeric material are uniformly dispersed throughout the matrix, larger amounts are often needed to get the desired effect, whereas if a lesser amount were to be concentrated in one thinner portion of the article, it could be possible to obtain the same effect.
Since the methods typically used to prepare polymer articles, such as, for example, melt processing and casting, involve intermediate states of the materials (such as melts, solutions, suspensions, and the like) in which the polymer and any additives are at least somewhat uniformly blended, the resulting products will generally comprise a uniform dispersion of the additives in the polymer matrix. It is possible to prepare articles in which additives are concentrated in a particular area by making laminates or the like, but these require the use of multiple processing steps, which can add complexity and cost.
It would thus be desirable to obtain monolithic polymeric composite articles in which additives were present in the form of the gradient, viz., that they were concentrated in a particular portion of the article.
U.S. Pat. No. 5,464,585 discloses a process for molding articles having a bulk material and an auxiliary material present in the bulk material as a fixed concentration strip or in a concentration gradient in the direction from the surface to the interior.